An electronic control for a conventional appliance, such as a typical domestic oven and/or cooktop appliance, requires a power supply. The power supply converts the line voltage available from a conventional residential outlet, such as 115 volts or 230 volts a.c., to a lower more useable voltage for electronic controls, such as a d.c. voltage between 3 volts and 48 volts, typically 12 volts d.c. There are several ways to transform the line voltage to the electronic control voltage including, but not limited to, an impedance dropping power supply and a transformer power supply.
Impedance dropping power supplies are typically well suited for domestic oven applications because they occupy a small amount of space and are lightweight. FIG. 1a illustrates a conventional resistive dropping power supply 10, and FIG. 1b illustrates a conventional reactive (capacitive) dropping power supply 12. These power supplies 10 and 12 are non-isolating. Typically for use with an oven, the dropping power supplies 10 and 12 transform the line voltage of 230 volts a.c. to the electronic control voltage of approximately 12 volts d.c. The power supplies 10 and 12 are "half wave" circuits which work 50% of the time or for half of the line cycle when the line voltage is positive. When the line voltage is positive, a dropping impedance, either resistor R1 or capacitor C1, drops the voltage from the line, and a zener diode Z1 clamps the voltage across the zener diode Z1 at the desired electronic control voltage. A diode D1 and filter capacitor C2 rectify the voltage. When the line voltage is negative, the zener diode Z1 acts as a normal diode and no current is passed to the rectifying diode D1 and capacitor C2. To provide the 12 volt d.c. electronic control voltage, the zener diodes Z1 have a 12 volt rating to clamp the voltage across the zener diode Z1 at 12 volts.
FIG. 1c illustrates a "fill wave" reactive dropping power supply 14. The "full wave" circuit includes a full rectifier bridge B1 and works 100% of the time for both positive and negative line voltages. When the line voltage is positive, a dropping impedance, a capacitor C1, drops the voltage from the line, the diode bridge B1 rectifies the voltage, and a zener diode Z1 clamps the voltage across the zener diode Z1 at the desired electronic control voltage. When the line voltage is negative, the bridge B1 rectifies the line voltage to allow the zener diode Z1 to clanp the voltage across the zener diode Z1 at the desired electronic control voltage, and the dropping impedance C1 drops the voltage to the line. A filter capacitor C2 filters or smoothes the voltage. To provide the 12 volt d.c. electronic control voltage, the zener diodes Z1 have a 12 volt rating to clamp the voltage across the zener diode Z1 at 12 volts. A "full wave" resistive dropping power supply (not shown) would be similar to the "full wave" capacitive dropping power supply 14 of FIG. 1c with the dropping capacitor C1 replaced by a resistor.
In the impedance dropping power supplies 10, 12 and 14, the "unused" current passes through the zener diodes Z1. If the required current is very low, such as 20 mA, the dropping impedance can be a resistor R1 as depicted in the resistive dropping power supply 10 without causing significant self heating problems. For higher currents, such as 80 mA, the dropping impedance needs to be non-dissipative, namely reactive such as depicted in the capacitor C1 dropping power supply 12; otherwise, if the resistor R1 were used, too much heat is generated by the power supply. For currents of the order of 200 mA, more capacitance must be added for the dropping impedance resulting in larger costs for the dropping impedance power supply. The currents described above are examples that apply to a 230 volt line supply. For 115 volt line supply, these currents can be much higher.
Using the transformer based power supply in ovens can be problematic. FIG. 2 illustrates a conventional transformer based power supply 16. To provide the 12 volt d.c. electronic control voltage, the transformer 18 has a rating to transform the 230 volts a.c. line voltage to approximately 8 volts a.c. the peak value of which is approximately 12 volts, and a diode D1 and filter capacitor C2 rectify the voltage. This circuit is half wave power supply; however, it may be made into a fill wave power supply with the use of a bridge rectifier. One problem with using the transformer power supply is the high ambient temperatures of the oven application. Transformers are difficult to design as their operating temperatures approach the temperature of their insulation limits. In addition, European safety agency requirements demand that the transformer be protected against straight secondary shorts which can require the addition of expensive thermal fuses. Additionally, the transformer tends to be very bulky and heavy creating packaging difficulties for the electronic control in the domestic oven appliance. Furthermore, transformers may be a very costly component for an appliance. Despite these shortcomings, transformers are typically used for current levels above 100 mA for oven power supplies because the additional capacitance required by the reactive dropping power supply 12 is more expensive than the transformer. Transformer based power supplies also have the advantage of being isolated if required.
To meet more strict electromagnetic interference ("EMI") regulations, modem appliances need EMI filters to prevent high frequency disturbances created by the appliance from affecting the power line supply. Typically, European laundry appliances require EMI filters to negate the effects of commutator switching on universal motors and the switching of various inductive loads. The increased use of electronics in the controls for appliances, such as triacs that may cause disturbances, contributes to the need for EMI filters. However, conventional ovens do not usually incorporate EMI filters because oven loads are almost entirely resistive. In addition, ovens are typically lower cost appliances, so manufacturers of ovens have avoided design changes that require the addition of EMI filters.
The domestic oven appliance includes fans, such as an internal convection cooking fan and a cooling fan. The cooling fan functions to keep the user accessible areas, such as the door handle, relatively cool and to keep the electronic controls below their maximum temperature rating. The conventional oven cooling and convection fans are typically single speed, low cost single-phase induction motor driven fans. These fans are rated at the line voltage, typically 230 volts a.c. The nominal rating of the typical cooling fan is approximately 20 W to 40 W, although due to the inherent inefficiency of the fans, very little of this power is useful. On the other hand, this very inefficiency makes the motor behave somewhat like a DC motor, in that its speed is highly dependent on the applied voltage. The current flow through the motor is a consequence of the applied voltage and the speed at which the motor is running. Since the motor is not very efficient, it appears to be very resistive, and therefore the speed and current are largely functions of the applied voltage. If the motor were more efficient, the relationship would be more complex. The typical current flow in the fans for ovens is approximately 150 mA to 250 mA.
Ovens equipped with electronic controls sometimes drive cooling and convection fans with triacs, rather than relays; however, this system has a very significant drawback. The triac driven loads control introduce objectionable power line disturbances requiring an expensive EMI filter.
Thus, it is desired to develop a low cost power supply circuit that transforms the line voltage to the electronic control voltage, that eliminates the need for a transformer, that permits variable speed fan operation, and minimizes electromagnetic interference.